This invention relates to brake systems, and more particularly to providing an anti-skid feature in master cylinder actuated braking systems.
Most land-based vehicles employ a single braking control for actuating the brakes. Aircraft, on the other hand, commonly utilize the brakes both for stopping the aircraft and for directional control and therefore employ independent left and right brake controls. Also, many aircraft have dual controls resulting in four different brake controls. Vehicle anti-lock braking systems are common in both land-based vehicles and aircraft. Typically, the anti-lock systems, if present, come as original equipment on the vehicle. A typical aircraft brake control system has many components to mount and connect hydraulically. It would typically need a communications interface to a central computer and often, as in brake by wire systems, needs to sense brake pedal travel. In addition, a typical brake control system normally operates on an aircraft with central hydraulics system having supply and return lines. These systems are relatively sophisticated, complex and expensive. Modification of a vehicle lacking anti-lock braking to incorporate an anti-lock feature is sufficiently complex and costly that it is rarely attempted. Moreover, since current aircraft anti-lock features are an integral part of the aircraft brakes, anti-lock failure is generally accompanied by brake failure.
A low cost, light weight, relatively simple anti-lock system suitable as original equipment or for retrofitting existing vehicles would be desirable, particularly for aircraft applications. It is also desirable to provide an aircraft braking system anti-lock feature where anti-lock failure is not accompanied by general failure of the braking system.
The present invention provides solutions to the above problems by adding anti-skid/anti-lock capability to aircraft equipped with master-cylinder or similar type stand-alone, manually operated braking systems, at a reasonable cost and without requiring major modification to the aircraft systems.
The present invention represents a departure in design and approach from existing aircraft anti-lock/anti-skid systems. The main differences are that the system is an add-in which augments the brake system functionality whereas existing systems are integral to the brake system. Existing systems require more extensive modification to the aircraft to add the functionality to existing units. The failure mode of the present invention is to revert to normal braking, thereby not depriving the pilot of braking ability. If there is a question as to the status or performance of the anti-lock portion of the system, it can be disabled by a single cockpit switch, having no further effect on the brakes.
The principle differences of the present invention as compared to other aircraft anti-skid units are lower cost, smaller size and parallel operation which defaults to normal braking while existing systems are integral and default to loss of function.
The invention comprises, in one form thereof, a technique for retrofitting an aircraft braking system of the type having two independently operable hydraulic braking circuits, each having a master cylinder for selectively supplying pressurized hydraulic fluid to a corresponding brake actuating mechanism to incorporate an anti-skid feature. The technique includes the introduction of two pair of valves, one pair into each of the two hydraulic braking circuits between the master cylinder and corresponding brake actuating mechanism. The first valve of each pair normally provides a fluid path between a master cylinder and corresponding actuating mechanism for normal braking and is operable during anti-skid operation against spring bias to block the fluid path between the master cylinder and actuating mechanism when a skid condition of an aircraft wheel is sensed by a system which indicates the aircraft wheel speeds. The second valve of each pair is normally closed and opens against spring bias to vent pressurized fluid from the corresponding actuating mechanism to a sump relieving the braking force during anti-skid operation. A pilot operable control and/or an anti-skid error sensing circuit may be provided for selectively disabling the electronic interconnection to fix all pairs of valves in their normal conditions hydraulically coupling the master cylinders to their respective brake actuating mechanisms thereby disabling the anti-skid feature. For enhanced reliability, for example, in the event one of the valves sticks in one position, two further pair of valves may be introduced, one further pair in parallel with a corresponding pair in each of the hydraulic circuits, and a pair of pressure actuated shuttle valves provided, each coupling one of each parallel valve pair to the corresponding actuating mechanism so as to select and pass the higher of two braking pressures applied to the shuttle valve inlets.
Also in general, a combined brake and anti-lock system for a wheeled vehicle has an operator controlled source of pressurized hydraulic fluid and a hydraulically actuated wheel rotation braking device which responds to applied hydraulic pressure to apply a braking force to a wheel of the vehicle to arrest wheel motion. A normally open hold valve is connected in the circuit between the source and the braking device for selectively opening and closing a fluid flow path from the source to the braking device. A normally closed bleed valve is connected in the circuit between the braking device and a low pressure hydraulic fluid return for selectively opening and closing a fluid flow path from the braking device to the return during anti-skid operation. A wheel speed sensing device provides an indication of the rotational speed of the wheel and a valve control is responsive to an indication of a skid indicative wheel speed irregularity from the sensing device to close the normally open hold valve and open the normally closed bleed valve to relieve braking force. There may be an operator control and/or malfunction sensor for disabling the valve control to allow anti-lock-free braking.
An advantage of the present invention is that no major modification of an existing brake system is required when incorporating an anti-lock feature.
Another advantage of the present invention is that both equipment and installation costs of adding an anti-lock feature are modest.
A further advantage is that the aircraft""s normal braking system is retained in its original capability and failure of the anti-lock system reverts to normal braking.